It depends who you ask...
I've seen L4M, L4(M), L4m and L4(m) from different sources. For what it's worth, the Blaw Knox drawings for them say L4(M), but that may be the whim of the draughtsman, and all the other lettering on the drawing is in capitals too. Also, the drawing is referring to a CEGB specification and I have no idea how that was bracketed or capitalised.
On the other hand, CEGB drawings for L6M (punctuate according to taste) are labelled L 6 M - no brackets, all caps and spaces between characters. Their drawings for L7C don't have spaces or brackets.
I don't think there ever was a pre-metric L4. The numbering of tower series seems to have been fairly random, certainly not sequential and with gaps.
I go for 'series' for L2, L3, L6 and so on. The appropriate field on the National Grid tower database is LINE_SERIE (presumably there's a field limit of 10 characters somewhere). Blaw Knox drawings say 'specification' on L2 and L4M drawings, which is probably strictly correct. CEGB for L6M, L7C and L12 have 'tower series'. Just to be different, LSTC use 'tower design' on their drawings for the proposed (and I think abandoned) L4S and L7H series. Given that CEGB used 'series', I think that's authoritative.
'Tower type' seems to be universal for D, D30, D60 and so on.
I think that may actually have clarified things a little. Answers to questions like that usually just make the subject murkier.
Ian